Nothing about Julio Teheran's performance this season merits a demotion, whether to the bullpen or the minor leagues. In fact, he has shown improvement just about every time out, most recently with 8 1/3 strong innings against the Twins. But the fact of the matter is that when Brandon Beachy returns, someone has to go.
Who else would it be? Paul Maholm? If his slide continues, his numbers could end up looking worse than Teheran's. But he doesn't have any bullpen experience, and it's not like he's going to the minors. By the time Beachy gets here, the Braves may prefer to move Teheran to the bullpen strictly to limit his innings.
I'm not saying you should drop Teheran in anticipation of Beachy's return. Someone else could go down between now and then, ending the dilemma. But you should prepare for life without him in Fantasy. My guess is it's rapidly approaching.